A device of the type described above is known from DE 42 24 047 A1. The device comprises a container shaped concentrically to a vertical axis. The container has a feed device positioned in the middle in the upper region. The feed device comprises a spinning chamber, a gravity pipe, and a trumpet-shaped diffusor, thus using the COANDA-effect. The device is used primarily to deposit sand out of a mixture of sand and liquid in a circular settlement tank. The sand depositing in the lower region of the tank depending on the gravity will be extracted by an extractor device designed as a screw conveyor device. The sand, especially out of sewage treatment plants, and the organic material adhere to each other more or less so that a large amount of the organic material is extracted and conveyed with the sand. Very fine distributed organic material only, which is in a suspending or floating state, is extracted via an overflow of the tank and is extracted together with the liquid. From the periodical Korrespondenz Abwasser, 1/94, pages 48 to 53 (Klinger/Barth "Entwicklung einer Sandrecyclinganlage fur Klaranlagen) a plant for recycling sand is known having a number of singular devices connected to each other. Thus, the material to be treated first is introduced into an oscillating screen in order to separate the larger parts. The material, having passed the oscillating screen, together with liquid is introduced into a classification device having an upward flow to separate the organic material from the sand. The introduction occurs via a hydrocyclone into the classification device. Finally, a dewatering screen with an excentrical drive is used. This effort of devices, tubes and pipes may be justified for complex separation purposes. For the separation of sand out of sewage treatment plants an effort as low as possible is necessary and an operation at low cost. The known plant is designed for relatively large amounts. It is difficult to use the plant for smaller quantities and/or to operate the plant intermittently. In addition, it is not possible to treat the mixtures of sand and liquid typical for sewage plants.